Various types of valve mechanisms such as globe valves, gate valves and rotary plate valves have been widely used for fluid flow control, also referred to herein as "throttling". Various types of rotary valves such as spherical plug or ball valves, straight rotary plug valves and tapered plug valves have also been frequently designed for fluid flow control activity. These types of rotary valves are not generally favored for flow control because they are frequently subject to severe vibration and noise during operation and they are also subject to a condition known as "windmilling" wherein the rotary valve element is subject to flow induced rotational forces which can oppose opening or closing actuator forces. This disclosure is particularly limited to rotatable plug type flow control valves because they are of rather compact design and because through the addition of noise abatement and anti-cavitation tim these valves can be quite effective from the standpoint of efficiently controlling the flow of high pressure fluids.
In many service conditions a principal problem with fluid flow control activity is the existence of significant pressure drop across the valve even when the valve is open to its maximum extent. This condition of significant pressure drop typically occurs because of partial obstruction of the flowway of the valve with flow controlling mechanisms or because of the unusual configuration of the flowway itself which is necessary to accomplish flow control activity. It is desirable to provide a throttling valve mechanism having a pressure drop in the full open position which is substantially that of a conventional full opening valve. To achieve minimal pressure drop across the valve a rotary spherical plug valve or ball valve is at times used because the ball can be positioned in a partially closed condition for flow control and in the full open position, offers minimal resistance to flow. The turbulence that is developed within the valve chamber and flowway of a ball vane in the partially closed flow controlling position thereof, especially under high pressure conditions, can cause severe vibration, can be exceptionally noisy and can induce severe cavitation. This disadvantage can be overcome by the addition of noise abatement and anti-cavitation trim which defines torturous paths through which the fluid flows. Further, forces tending to rotate the ball of a conventional ball valve past a critical position, especially when the critical position is more toward the closed position of the valve, can become reversed and can cause windmilling of the rotary valve element which at times opposes the actuator force and at times adds to the actuator force or requires the actuator to apply a braking force to retard the flow induced rotational force. A valve actuator for these types of flow control valves must therefore be capable of inducing a force for valve opening and be capable of retarding the valve closing force after the valve ball has been rotated past the critical position. Because of these disadvantages the use of conventional ball valves for purposes of throttling activity is not generally favored. It has been determined however that the addition of means defining a torturous path through the flow controlling passages of the valve and the development of a pressure drop as the fluid flows through the valve minimizes noise, vibration and cavitation and permits the advantages of these valves to be used.
Many high pressure rotatable flow control valves such as ball valves have been employed which incorporate valve body flow passages and valve ball flowways of differing dimension so as to permit the development of a pressure differential across the valve plug or ball particularly for the purpose of anti-cavitation and noise minimization. When valves of this type are utilized for flow controlling purposes high velocity flow of fluid through a restricted orifice can induce significant turbulence that causes the valve to vibrate while in service. At times depending on flow conditions this vibration can be quite violent. Additionally this extremely high pressure flow of fluid through a restricted orifice can develop significant noise that renders these types of valves quite unsatisfactory. It is well known that cavitation activity within a control valve is a cause of rapid deterioration of internal valve components and internal valve surfaces. It is desirable in almost all cases to place significant design emphasis on valve systems for minimizing cavitation during valve operation. Various types of anti-cavitation and noise abatement mechanisms, typically referred to as "trim" have been developed for utilization in flow control valves to establish torturous paths for controlled fluid flow and develop a designed pressure drop as the fluid flows through the valve to thus minimize the detrimental effects of fluid cavitation and high velocity fluid noise as well as valve vibration.
In the pipeline industry the inside walls of the pipe through which the fluid flows can become contaminated by line scale, sand and other such debris. Additionally, deposits of material from the flowing fluid can adhere to inside; surfaces of the flow line and any valves that are employed to control the flow of the fluid. For this reason various mechanical cleaning devices such as pigs, balls, scrapers and the like are passed through a flow line under the influence of the flowing fluid to accomplish periodic line cleaning. In some cases it may be desirable to provide for mechanical cleaning of flow lines that are controlled by throttling valves. For the reason that few flow control valves define straight through passages and the flow passages of many flow control valves are at least partially obstructed by noise abatement and anti-cavitation trim, the use of line cleaning devices in throttled flow lines is not ordinarily a consideration. It is desirable therefore to provide a flow control valve mechanism that can permit such line cleaning devices be capable of being used. It is also desirable to provide a rotatable flow control valve mechanism having the capability of achieving a straight through flow passage at all positions thereof from virtually closed to full open so as to permit unobstructed flow of fluid in the fully open condition of the valve to provide for minimal pressure drop and to provide for maximum flow at any given pressure condition. Most flow control valves are designed so that any normal wear or erosion that is induced during use will occur on replaceable valve parts. Thus, when repair is necessary the valve mechanism can be quickly disassembled and the replaceable worn components can be removed and replaced. Especially in the case of large flow control valves for pipelines and the like it is desirable to provide the capability for repair of these valves in place. This feature permits the repair operation to be expedited so that the down time of the flow line can be minimized and the cost of repair can also be minimized. It is also desirable therefore to provide a rotatable plug type flow control valve mechanism having the capability of being repaired while remaining in the line. This is typically accomplished by providing a top entry type plug valve or providing a plug valve capsule that is removable from a location between connecting flanges of the flow line.
As mentioned above, few flow control valves are provided with good rangeability of flow control, i.e., the ability for the control of flow to be varied over a wide range responsive to selected positions of the rotary flow control element. Thus, when flow conditions of a flow line change, it is often necessary to replace the flow controlling trim to accommodate the changed conditions. It is desirable to provide a rotary flow control valve having wide rangeability so that it is seldom necessary to shut down the flow line for trim package replacement. At times the flow controlling trim of control valves can become partially blocked with accumulated debris such that desired flow control cannot be accomplished. If the valve mechanism has a wide range of flow control, i.e., good rangeability, this problem can often be accommodated by simply adjusting the flow control position of the valve element.
Rotatable plug valves have been developed for flow control service having straight, tapered or spherical valve plug elements that permit flow control across a wide range of flow conditions and also provide the valve with full opening capability. U.S. Pat. No. 4,212,321 of Hulsey is an example of a spherical plug type full control valve of this nature. Though capable of providing exceptional flow control and having excellent rangeability, the valve of the Hulsey patent requires an axis of valve rotation that is inclined with respect to the longitudinal axis of the flowway, which prevents its effective use for valves that are intended for inline repairability. Additionally, rotational movement of the valve ball between fully open and fully closed positions typically requires valve ball rotation significantly exceeding 90.degree.. Thus, valve operators for opening closing and selective positioning of the valve ball are typically of special design and add further to the expense of the valve mechanism. It is desirable to provide a rotary flow control valve having a rotary valve element provided with a flowing therethrough and flow control slots which are open to the flowway and intersect the annular surface of the valve element. The flow control slots are of contoured configuration and defined by a ratio of flowway diameter to ball diameter to be in the range of from 2.0 to 1 to 4.0 to 1 . This ratio defines the maximum length of the flow control slot. The 2.0 to 1 ratio would have a shorter flow control slot than a 4.0 to 1. A ratio range of from 2.3 to 3.2 would be considered optimum in comparison with the diameter of the valve element which establishes complete shutoff and full opening in rotary valve movement of about 90.degree.. These features permit the rotatable flow control valve mechanism to have the capability of achieving movement between its fully open and fully closed positions in valve plug rotation of about 90.degree. while at the same time minimizing the overall dimension of the valve element and thus the overall dimension of the valve mechanism. It is also desirable to provide a rotatable plug type flow control valve having a valve stem that is oriented in normal relation to the axis of the flow passage defined by the valve body to thus minimize the complexity of the valve mechanism, to provide for controlled movement by standard valve actuators, and to permit it to be manufactured in a top entry, in line repairable, design if desired.
When flow control valves of a general nature are provided for controlling the flow of high pressure fluid it is typical for the anti-cavitation and noise minimization trim to be different at the inlet and outlet ports of the valve so that the valve becomes basically unidirectional. It is desirable from the standpoint of the present invention to provide a spherical rotational flow control valve having a quarter turn valve plug with anti-cavitation and noise abatement trim within the throttling passages of the valve plug and wherein the flow controlling trim at the inlet and outlet flow control passages may be substantially the same or different as desired.